Such a hydraulic control mechanism is shown and described, instance, in patent specification EP-B-0133330. More particularly still, the invention applies to a control system for a transmission having a plurality of rollers, typically six, each mounted on an individual ram and all of which must transmit the same ratio at any one time. One of those rollers, known in the art as the "master roller", is mounted on a double-acting ram the two cylinders of which are in direct connection with the control valves by which the two fluid pressures are controlled, while the fluid in the corresponding ram cylinders of each of the other rollers (known as the "slave rollers") is simply in communication with the cylinders of the master roller and so at a common pressure. A control circuit for such a combination of the master and slave rollers is also described in specification EP-B-0133330.
When a transmission with such a control system is working for the time being at normal rates of ratio change, the roller control rams will all be moving slowly within their respective cylinders. The separate flows of fluid down the two sides of the system-that is to say a first side including all the cylinders at one end of the respective rams, and a second side including all the cylinders at the other end-need only be sufficient to maintain the cylinders full on that side of the system where the cylinder volumes are increasing. A flow rate of a maximum of say 10-15 L/min is typical for vehicle transmissions, so that the hydraulic power source of the system need only comprise two small pumps each capable of delivering that flow, or alternatively a single pump capable of delivering say 20-30 L/min and fitted with a flow divider. However, when such a transmission undergoes a very rapid change of ratio, for instance when fitted to a vehicle which is approaching wheel-lock during emergency braking, all the roller control rams move rapidly in the same direction. The volume avalilable for fluid in the cylinders on one side of the system therefore increases rapidly, requiring a power source capable of delivering a considerable flow in order to maintain a reasonable minimum pressure within the cylinders or, in an extreme case, to avoid cavitation within them. To meet such conditions, it has therefore been customary to provide such hydraulic control systems with power sources far larger and more powerful than is necessary while the rams are either stationary or moving slowly, as is the case during most of the working life of the transmission.